The present invention relates generally to two-part casting molds for manufacturing grid plates for lead batteries, and in particular, to casting mold inserts for use with an external, metallic mold holder.
Casting molds of this type have only recently become known. For example, EP-PS No. 65,996 discloses the preparation of a ceramic coating for a metallic casting mold (made of cast iron) to eliminate the previously required, and generally cumbersome powdering with cork flour. In connection with this disclosure, there is described the formation of interchangeable mold linings (i.e., as separate inserts) formed of ceramic materials. The metallic casting mold acts as a holder for the ceramic mold inserts.
Both the coating of metallic casting molds with cork flour or a black wash, which is still common practice, as well as the lining of cast-iron mold halves with a ceramic material, have as their primary task thermal insulation of the lead melt from the metallic mold which receives it, while maintaining good thermal conductivity. This serves to prevent premature solidification of the lead melt during the pouring procedure, before all of the cavities of the mold have been filled. These measures also allow air to be displaced during the filling process and to escape from the mold cavity, as well as to assure satisfactory removal of the solidified casting from the mold due to its non-wettability. The requirements imposed on an insert having a shaped parting layer consequently include high thermal insulation capacity along with high temperature stability, porosity and non-wettability by the molten lead (or lead alloys).
Ceramic inserts have been found to generally meet these requirements. The stability in shape which is inherent in ceramic materials is especially advantageous in connection with the casting of very fine grid plates with filigree-like structures. Long useful-lives corresponding to a few thousand castings can be expected with such inserts. However, such inserts are manufactured from individual ceramic foils which are cut, laminated together, pressed by a heatable die, and finally sintered above 1000.degree. C., and are therefore rather expensive.